After a well has been drilled, it is sometimes very helpful to run certain test tools in the open hole. Ordinarily, the drilling process is stopped with the hole filled with drilling mud. Moreover, a mud cake is formed against the side wall of the open hole. The mud cake more or less stratifies solids in the mud. When a test instrument is run into a well in this condition, the instrument is typically lowered to a depth where a certain tests are performed. Ordinarily, the instrument is lowered into the well and then is retrieved. The test instrument is lowered to a specified depth for performing a test. There are certain tests that require that the test tool be lowered and held at a specific elevation. Indeed, there are certain test instruments such as a multi-test tool which momentarily attach to the formation at a specified depth; the tool is operated at that elevation. When operated, it is held at a specified elevation for a period of time which may range from minutes to hours.
During the test, the test instrument is typicallly decentralized. Moreover, the wire line which supports the instrument thereabove is also decentralized. Inevitably, it is not possible to drill a well sufficiently straight and maintain the supportive wire line in the center of the open hole. Usually, the line falls over to one side or the other of the well borehole. When the wire line does move to the side, it cuts through the mud cake, and may even cut a key seat. It should be kept in mind that the open hole may well be many inches in diameter while the wire line is typically of small diameter. For instance, one industry standard wire line has a diameter of 7/16 inches.
The well will typically pass through several strata. Some of the formations may be completely impervious. That is, they may have no porosity in the rock which comprises the formation. However, some formations penetrated by the open hole may have porous materials; such porous formations may have an internal pressure which is sufficiently low that fluid migrates from the open hole into the rock formation. Migrating filtrate from the mud may flow into the zone and leave mud solids adjacent to the particular zone. In the event the supportive wire line cuts through the mud cake and lies against the side wall, there is a chance of pressure differential sticking of the cable.
Even though the cable may be quite small in diameter, it nevertheless has a specific width. This width (when multiplied by the length of cable adjacent to the low pressure zone) yields a surface area which is subject to pressure differential sticking of the wire line against the porous zone. Moreover, the flow of mud filtrate into the zone may sustain a pressure differential between the pressure in the open hole and the pressure within the zone. This creates a lateral force holding the logging cable against the key seat area and differential sticking may well occur instantly.
In the past, it has been policy to retard this by continuously raising and lowering the wire line. As a rule of thumb, the wire line is "yo-yoed" up and down approximately four feet per thousand feet of depth. Thus, if the tool at the moment is ten thousand feet deep, the wire line is yo-yoed approximately forty feet. That is, the wire line is relaxed by reeling out forty feet and then reeling in forty feet.
At the bottom end of the wire line, the test tool is fixed. Thus, the forty feet of slack typically collects near the top of the hole, and the wire line in the lower portions of the hole (just above the test instrument) is primarily static, not moving, and more readily subject to pressure differential sticking. When it sticks, it is essential to pull hard on the wire line to retrieve the equipment. If the wire line breaks, it is subsequently necessary to engage in a fishing job which is expensive and delays completion of the well.
Typically, sticking occurs just in the region above the tool because the wire line there is not moving, subject to pressure differential sticking, and not benefited by the yo-yoing of the wire line from the well head.
The apparatus of this disclosure enables the wire line to be yo-yoed from the surface and yet enables that motion to be imparted to the wire line connected to the test instrument; however, the test instrument itself is not moved. Thus, it is stationary while the wire line above is continuously moved to reduce the risk of pressure differential sticking.
With this in mind, the present apparatus is summarized as an extendable connector structure formed of two concentric sleeves and attached above a test tool. The apparatus utilizes concentric sleeves which telescope. They can be moved by reciprocating motion of the wire line. The wire line often includes electrical conductors and a suitable coiled electrical conductor is also included in this apparatus to enable the wire line conductors to communicate with the tool below the present invention.